材料：

When a clean bill of lading has been issued，estoppel arises or，in other words，the carrier is prevented from proving，as against a third party relying on the clean bill of lading，that there was any thing wrong．In the case of inherent vice，however，which is a natural defect in the thing shipped，the carrier is not estopped by his clean bill of lading，because a clean bill of lading，in stating that a certain cargo is on board，gives sufficient notice that this cargo，in the natural course of events，may have certain qualities or defects which all similar cargo normally has．For example，a cargo of flour will shrink slightly，and this fact does not have to be noted on the face of the bill of lading． 

Similarly，where cargo is unfit to withstand the ordinary incidents of the contractual voyage contemplated by the parties，owing to some inherent vice or hidden defect presented within it，the carrier is not estopped by the clean bill of lading from asserting the inherent defect exception，provided that the damage has not been aggravated by any conduct of the carrier or those for whom he is responsible． 

The carrier issuing a clean bill of lading is only bound by his statements as to the outward condition of the cargo and is therefore not estopped from proving inherent vice or hidden defect when that vice or defect was not apparent at the time of shipment．In other words，a clean bill of lading does not necessarily fulfill the shipper's burden of proof in respect to inherent vice or hidden defect． 

问题：

The slight shrinkage of flour is ________．